7 binders cost $7.21. Which equation would help determine the cost of 15 binders?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 7 binders. We want to know the cost of 15 binders. We can write the numbers of binders as a proportion: $\dfrac{7}{15}$ We know 7 binders costs $7.21. We can let $x$ represent the unknown cost of 15 binders. The proportion of these costs can be expressed as: $\dfrac{\$7.21}{x}$ The cost changes along with the number of binders purchased, and so the two proportions are equivalent. $\dfrac{7}{15} = \dfrac{\$7.21}{x}$